If-Expression.md

title	parent
If Expression	Control-Related-Expressions.md

Syntax

if_expr ::= 'if' expr_c block_expr_t ['else' block_expr_f]

Note that in contrast to most imperative language like C/C++ or Java, an if-construct is not a statement but an expression and, thus, yields a value. The else clause is optional.

Typing

The condition expr_c of an if-expression must be of type bool. Both block_expr_then and block_expr_else must be of the same type or of type <no-return>. If the optional else-clause is elided, its type is unit: ().

Semantics

If expr_c evaluates to true, the whole expression evaluates to block_expr_t. Otherwise, it evaluates to block_expr_f.

Note that an if-expression is lazy. This means, that block_expr_t or block_expr_f, respectively, are evaluated after the whole if-expression has been evaluated.

Examples

fn main(x: i32) -> i32 {
   let y = if x == 0 { 23 } else { 42 };

   if y == 23 {
       1
   } else {
       0
   }
}